The spinal column is a complex system of bones and connective tissues that provide support for the human body and protection for the spinal cord and nerves. The adult spine includes an upper portion and a lower portion. The upper portion contains twenty-four discrete bones, which are subdivided into three areas including seven cervical vertebrae, twelve thoracic vertebrae, and five lumbar vertebrae. The lower portion includes the sacral and coccygeal bones. The cylindrical shaped bones, called vertebral bodies, progressively increase in size from the upper portion downwards to the lower portion.
An intervertebral disc along with two posterior facet joints cushion and dampen the various translational and rotational forces exerted upon the spinal column. The intervertebral disc is a spacer located between two vertebral bodies. The facets provide stability to the posterior portion of adjacent vertebrae. The spinal cord is housed in the canal of the vertebral bodies. It is protected posteriorly by the lamina. The lamina is a curved surface with three main protrusions. Two transverse processes extend laterally from the lamina, while the spinous process extends caudally and posteriorly. The vertebral bodies and lamina are connected by a bone bridge called the pedicle.
The spine is a flexible structure capable of a large range of motion. There are various disorders, diseases, and types of injury, which restrict the range of motion of the spine or interfere with important elements of the nervous system. The problems include, but are not limited to, scoliosis, kyphosis, excessive lordosis, spondylolisthesis, slipped or ruptured discs, degenerative disc disease, vertebral body fracture, and tumors. Persons suffering from any of the above conditions typically experience extreme and/or debilitating pain, and often times diminished nerve function. These conditions and their treatments can be further complicated if the patient is suffering from osteoporosis, or bone tissue thinning and loss of bone density.
Spinal fixation devices are widely employed in surgical processes for correcting spinal injuries and diseases. When the disc has degenerated to the point of requiring removal, there are a variety of interbody implants that may be utilized to take the place of the disc. These include interbody spacers, metal cages, and cadaver and human bone implants. In order to facilitate stabilizing the spine and keeping the interbody in position, other implants are commonly employed, such as bone screws and spinal rods. Depending on the pathology and treatment, a surgeon will select the appropriate spinal rod material and size, specifically, the cross-sectional diameter of the spinal rod.
Subsequent to surgical treatment of a patient's spine is the occurrence of proximal junctional kyphosis (PJK). PJK is a spinal deformity condition that may occur if the lumbar lordosis and thoracic kyphosis are not properly restored post-surgery. PJK may also be caused by the accelerated degeneration of the joint capsules and smaller articular processes at one or two levels above or below the junctional region. PJK appears at or above the cranial-most thoracic level treated. Even though PJK most commonly occurs in the thoracic region of the spine, it can also occur in various spinal regions and may occur above or below the instrument levels and may impact the next adjacent level or two that is not instrumented. This type of failure is called adjacent level failure. Symptoms of PJK and adjacent level failure include pain, neurological deficit, ambulatory difficulty, and poor maintenance of sagittal balance. For patients that demonstrate these symptoms, often the only treatment is an additional surgery. The incidence rate of PJK may be upward of 50% of long construct, instrumented fusion cases. Factors contributing to this condition are the end vertebrae selection, facet violation, weakened structural support due to significant soft tissue disruption, extensive junctional paraspinal musculature dissection, and loss of integrity of the posterior tension band.
One thought to address the incidence of PJK is to decrease the structural rigidity of the construct at the top of the construct just below the proximal junction, thereby providing a transition from the relatively stiff instrumented spine to the more flexible, non-instrumented spine to minimize facet capsule and muscle disruption.
Spinal rods are typically made of cobalt chrome, stainless steel, or titanium alloy. However, in order to transition to a less stiff construct at the top, other less rigid materials may be employed to provide the desired stiffness.
A continuing need exists for an improved device, an improved system, and an improved method for performing spine surgery that does not create additional morbidity post-surgical treatment.